Most automotive and heavy-duty vehicles use engine starter motors with internal gear trains. The internal gear trains are typically planetary configurations. Planetary gearing is a gear system that consists of an outer gear referred to as an annulus, one or more planet gears, and a central gear referred to as a sun gear. Typically, the planet gears are mounted on a planetary gear carrier and are configured to rotate around the sun gear. The planet gears are meshingly engaged with a geared opening of the annulus. The annulus is typically held in a stationary position within a housing of the starter motor.
The gear train transfers rotation of an electric motor of the starter motor to an overrun clutch of the starter motor and then to a pinion gear of the starter motor. In particular, an input of the gear train, which is typically the sun gear, is connected to an output shaft of the electric motor. An output of the gear train, which is typically the planetary gear carrier, is connected to an input of the overrun clutch. An output of the overrun clutch is connected to the pinion gear. Rotation of the output shaft of the electric motor results in rotation of the sun gear. The rotating sun gear causes the planet gears and the planetary gear carrier to rotate. The overrun clutch transfers rotation of the plenary gear carrier to the pinion gear. The pinion gear is slidably mounted on a pinion shaft, and is movable between an engaged position and a disengaged position.
To start an engine with the typical starter motor, the pinion gear is moved to the engaged position, in which the pinion gear becomes meshingly engaged with a geared portion of a flywheel of the engine. Next or at the same time, the electric motor is activated, which causes the pinion gear and the flywheel to rotate. The rotating flywheel puts the engine pistons into motion, which typically causes the engine to start. When the engine does start, the flywheel begins to rotate at a rotational rate that is greater than the rotational rate of the pinion gear, which causes the overrun clutch to decouple the pinion gear from the output of the gear train. This prevents damage to the gear train, which may occur as a result of the rapidly rotating flywheel. The pinion gear is moved to the disengaged position after the engine is started.
When the pinion gear is engaged with the flywheel and is rotating the flywheel, the engine loads the starter motor with a pulsating torque that is a result of the pistons moving within the engine cylinders, among other moving engine parts. The pulsating torque is typically less than the stall torque, a term that refers to the magnitude of torque that causes the output shaft of the electric motor to stop rotating. The engine, however, may load the starter motor with a torque that is much greater in magnitude than the stall torque during certain engine events. These engine events may include engine backfire, hydraulic lock-up, or engagement of the pinion gear with the flywheel after the engine is already started. For example, hydraulic lock-up may result in a torque that is about five hundred percent (500%) to six hundred percent (600%) of the stall torque. The high torque is primarily caused by kinetic energy stored in the output shaft of the electric motor, which is then converted to strain energy upon rapid deceleration of the output shaft.
Vehicle manufacturers require that the starter motor should not fail or cause failure of other engine components as a result of the high-torque engine events described above. To meet this requirement, starter motor manufacturers design starter motor components to withstand a torque in excess of the stall torque. This often results in starter motor components being larger, heavier, or made from more robust and expensive materials than if the components were only required to withstand the torque encountered during normal engine operation.
Therefore, it is desirable to provide a starter motor that limits the torque internal to the starter motor so that the starter motor can be smaller, lighter, and less expensive to manufacture.